France and Iran voiced concern over escalating violence in Syria on Tuesday, echoing warnings from the United States and Russia as fighting near the city of Aleppo put more pressure on a fragile truce agreement.

The already widely violated "cessation of hostilities" agreement brokered by Russia and the United States has been strained to breaking point by an upsurge in fighting between Syrian government forces and rebels near Aleppo.

The escalation underlines the already bleak outlook for peace talks set to reconvene this week in Geneva. The United Nations says the talks will resume on Wednesday. The government delegation has said it is ready to join the talks from Friday.

With President Bashar al-Assad buoyed by Russian and Iranian military support, the Damascus government is due to hold parliamentary elections on Wednesday, a vote seen by Assad's opponents as illegitimate and provocative.

Iran said an increase in ceasefire violations could harm the political process a day after Russia said it had asked the United States to stop a mobilization of militants near Aleppo, Syria's biggest city until the conflict erupted in 2011.

Iranian Deputy Foreign Minister Hossein Amir-Abdollahian, speaking after a meeting with U.N. Special Envoy Staffan de Mistura in Tehran, blamed the "increasing activities of armed groups" for the violations.

France, which backs the opposition, also expressed concern, but blamed the other side. "It warns that the impact of the regime and its allies' offensives around Aleppo and Eastern Ghouta are a threat to the cessation of hostilities," government spokesman Romain Nadal said. The Eastern Ghouta is an opposition-held area near Damascus.

Syria's civil war has killed more than 250,000 people, created the world's worst refugee crisis, allowed for the rise of Islamic State and drawn in regional and international powers. The intervention of Russia swung the war in Assad's favor.

Washington "very, very concerned"

The United States, which also backs rebels fighting Assad, on Monday said it was "very, very concerned" about increased violence and blamed the Syrian government for the vast majority of truce violations.

Both the government and a large number of rebel groups had pledged to respect the cessation of hostilities agreed in February with the aim of allowing a resumption of diplomacy towards ending the five-year-long war. Jihadist groups including the Nusra Front and Islamic State were not part of the deal.

A senior official close to the Syrian government said the truce had effectively collapsed.

"On the ground the truce does not exist," said the official, who is not Syrian and declined to be named because he was giving a personal assessment. "The level of tension in Syria will increase in the coming months."

The eruption of fighting on the front lines south of Aleppo marks the most serious challenge yet to the truce.

The Syrian Observatory for Human Rights, a British-based organization that tracks the war, said dozens of government fighters had been killed in a big offensive to take the town of Telat al-Eis near the Aleppo-Damascus highway on Tuesday.

A rebel fighting in the area said the assault launched at dawn was backed by Russian air strikes and Iranian militias, adding that the attackers had suffered heavy losses. The Syrian military could not be reached for comment.

Iranian Revolutionary Guards and Lebanon's Hezbollah have both deployed in the southern Aleppo area in support of the government, while the Nusra Front is also fighting in close proximity to other rebels.

Children play near upright buses barricading a street, which serve as protection from snipers loyal to Syria's President Bashar al-Assad, in Aleppo's rebel-controlled Bustan al-Qasr neighborhood, Syria April 6, 2016. Photo: Reuters/Abdalrhman Ismail

The Syrian prime minister was quoted on Sunday as saying government forces were preparing a major operation in the region with Russian support.

Further south in Homs province, Russia said one of its attack helicopters had crashed in the early hours of Tuesday, killing both pilots. It said the helicopter had not been shot down and the cause of the crash was being investigated.

"Provocative" election

De Mistura, speaking in Tehran, said he and Amir-Abdollahian had agreed on the importance of the cessation continuing, that aid should reach every Syrian and that "a political process leading to a political transition is now crucially urgent".

De Mistura, whose two predecessors quit, has said he wants the next round of Geneva talks to be "quite concrete" in leading towards a political transition.

Ahead of the first round of talks, Damascus had ruled out any discussion of the presidency, calling it a red line.

A senior Iranian official on Saturday rejected what he described as a U.S. request for Tehran's help to make Assad leave power, saying he should serve out his term and be allowed to run in a presidential election "as any Syrian".

Some members of the main Syrian opposition alliance, the High Negotiations Committee (HNC), arrived in Geneva on Tuesday, and U.N. spokesman in Geneva Ahmad Fawzi said the talks were expected to begin on Wednesday.

De Mistura is working according to a U.N. Security Council resolution approved in December that sets out a political process including elections after the establishment of "credible" governance and the approval of a new constitution.

The Syrian government says it is holding Wednesday's elections in line with the existing timetable that requires a vote every four years. Russia has said the vote does not go against the peace talks and is in line with the constitution.

French President Francois Hollande last month, however, said the idea was provocative and "totally unrealistic".